1. Field of the Invention
This invention is a high frequency high bandwidth transversal filter. One application of this circuit is a high data rate matched filter.
2. Description of the Prior Art
A transversal filter samples a signal at several intervals of time, weights the samples, and sums them to produce an output signal. A block diagram of such a filter is shown in FIG. 2. An input signal is supplied at 1 to a delay line 2. Symmetrically deposed about a central tap 3 of delay line 2 are a plurality of pairs N of secondary taps 4 and 5, 6 and 7, and 8 and 9, separated from the central tap 3 at multiples of a length corresponding to a delay of time T. (FIG. 2 illustrates the special case where N, an arbitrary number, is equal to 3.) These taps are connected to weighting amplifiers 10, 11, 12, 13, 14, 15, and 16, respectively. The weighting factors associated with these amplifiers are denoted as A(0), A(-1), A(1), A(-2), A(2), A(-N), and A(N), respectively. The output signal emerges at 18. The transfer function of such a filter is ##EQU1## where .omega. is the frequency, a(K) are the weighting factors, and T is the delay between taps.
A prior art search was conducted and the following patents were uncovered:
U.S. pat. No. 3,768,046 is a precision distributed parameter delay line. This patent uses a coaxial or microstrip transmission line for a delay and a stub tuned input to improve the rise time. This circuit is not a filter. The use of a coaxial or microstrip transmission line for a delay is well known. A tuned stub is not used or needed in the present invention.
U.S. Pat. No. 2,915,716 is a microstrip filter. The geometry of the microstrip is modified to provide different lumped inductances and capacitances. This filter is the microwave equivalent of a lumped LC filter. It is a passive filter of a class having an Infinite Impulse Response (IIR). My invention is a transversal filter, using distributed inductances and capacitances of a class having a Finite Impulse Response (FIR).
U.S. Pat. No. 2,759,044 is a circuit for beam aperture correction in horizontal and vertical directions. A transversal filter is used; however, this circuit is not applicable to high frequency operation. The taps on the delay line upset the input impedance of the filter unlike in my invention. This causes phase shifts, attenuation, reflections, and other matching problems at high frequencies, which distort the signal and limit the bandwidth over which this circuit can be used. Furthermore, the junction summing circuit restricts the bandwidth and precludes the additional amount of delay which can be provided by my distributed summing circuit.
U.S. Pat. No. 3,688,194 is a waveform transient measuring circuit and method. The characteristics of the line are changed to compensate for the attached network. The circuit was not intended as a filter nor can it be used as one.
U.S. Pat. No. 3,611,201 is a carrier transversal equalizer. Couplers are used to tap a signal off a transmission line. These signals are then passively attenuated, delayed, and phase shifted. The resulting signals are coupled onto an output line. While these couplers can be constructed so as not to disturb the impedance of the input lines, they are highly lossy, limit the bandwidth, and restrict the filter to intermediate frequency applications. My invention can provide gain and operates from D.C. to frequencies limited only by the transistor characteristics.
U.S. Pat. Nos. 4,035,628 and 3,543,009 were uncovered as secondary references.